Multiple Access Interference Suppression Research Articles

A Space-Time Signal Decomposition Algorithm for Downlink MIMO DS-CDMA Receivers

We propose a dimension reduction algorithm for the receiver of the downlink of direct-sequence code-division multiple access (DS-CDMA) systems in which both the transmitters and the receivers employ antenna arrays of multiple elements. To estimate the high order channel parameters, we develop a layered architecture using dimension-reduced parameter estimation algorithms to estimate the frequency-selective multipath channels. In the proposed architecture, to exploit the space-time geometric characteristics of multipath channels, spatial beamformers and constrained (or unconstrained) temporal filters are adopted for clustered-multipath grouping and path isolation. In conjunction with the multiple access interference (MAI) suppression techniques, the proposed architecture jointly estimates the direction of arrivals, propagation delays, and fading amplitudes of the downlink fading multipaths. With the outputs of the proposed architecture, the signals of interest can then be naturally detected by using path-wise maximum ratio combining. Compared to the traditional techniques, such as the Joint-Angle-and-Delay-Estimation (JADE) algorithm for DOA-delay joint estimation and the space-time minimum mean square error (ST-MMSE) algorithm for signal detection, computer simulations show that the proposed algorithm substantially mitigate the computational complexity at the expense of only slight performance degradation.

Subspace-Based Schemes for NBI and MAI Suppression in DS-CDMA Communications

This paper deals with narrowband interference (NBI) and multiple access interference (MAI) suppression based on subspace-based approaches for direct-sequence code-division multiple-access (DS-CDMA) systems. First, the signal subspace-based and the interference subspace-based techniques are developed to estimate the noisy DS-CDMA signal and the interference signal, respectively. In conjunction with a modified multiple constrained minimum variance multiuser detector, we develop two efficient schemes to deal with the NBI and MAI that do not need any prior knowledge about the characteristics of the interference and specific training sequence. Next, an adaptive implementation based on a signal subspace tracking algorithm and a recursive least mean square algorithm is also developed. Several computer simulation examples in terms of bit error rate are provided for illustrating the effectiveness of the proposed schemes.

Performance analysis of RLS linearly constrained constant modulus algorithm for multiuser detection

The linearly constrained constant modulus algorithm (LCCMA) is a blind multiuser detector (MUD) solution to multiple access interference (MAI) suppression that is widely investigated in direct-sequence code division (DS-CDMA) systems. However, the conventional CMA based on the stochastic gradient descent (SGD) has slow convergence speed. Our research introduces an approximation of recursive least square (RLS) into LCCMA for better convergence speed in DS-CDMA system and quantifies the performance of blind adaptive filter based on RLS-LCCMA in both a static and a time-varying channel. In this investigation, we derive the expressions for the excess mean-square error (EMSE) of the MUD with a framework called feedback approach, and further obtain a relationship between the step size of SGD-LCCMA and the forgetting factor of RLS-LCCMA. Eventually, simulation results show the advantage of RLS-LCCMA and verify the performance analysis of the algorithm.

Multiple-Access Interference Suppression for Interleaved OFDMA System Uplink

In this paper, the issue of multiple-access interference (MAI) suppression for the uplink in an interleaved orthogonal frequency-division multiple-access (OFDMA) system is investigated. In such a system, a carrier frequency offset (CFO) disrupts the orthogonality between the subcarriers and gives rise to MAI among users. Based on the signature vector formulated for each user, we propose a novel detector that performs MAI suppression before CFO compensation and fast Fourier transform (FFT) demodulation. Subspace zero-forcing and minimum mean square error (MMSE) techniques are then developed to suppress MAI. The proposed scheme is shown to become almost MAI free, provided that the CFO estimation is accurate enough. From the obtained simulation results, the proposed scheme is also found to be able to enhance the system performance at low complexity.

Expanded decorrelating detector with reduced noise enhancement for multipath frequency-selective fading channels

A novel hybrid multiuser detection scheme that jointly uses linear and nonlinear interference suppression techniques is developed for high-speed direct-sequence code-division multiple-access communications in multipath frequency-selective fading channels. The detector detects signals in a symbol-by-symbol style. Conventional decorrelating detectors suffer from the noise enhancement problem, which becomes more serious for dispersive multipath channels. The proposed detector uses interference cancellation technology to reduce the rank of the expanded signal subspace and hence it preserves the advantages of the expanded decorrelating detector in terms of complete multiple access interference and intersymbol interference suppression and meanwhile avoids its disadvantage in terms of noise enhancement. Computer simulation shows clear superiority of the new detector to other existing methods.

Performance analysis of non-data aided SBIB receivers for CDMA systems

In this paper, a non-data aided desired signal-blocked interference-blocked (SBIB) receiver with enhanced multiple access interference (MAI) suppression is proposed for direct-sequence code-division multiple access (DS-CDMA) systems over a multipath fading channel. Without using pre-known data, the proposed receiver can offer a similar performance as a non-blind maximum signal-to-interference-plus-noise ratio (MSINR) receiver, in which both the interference-plus-noise correlation matrix and the channel estimation are assumed to be correct. Performance analysis is carried out to examine the efficacy of the proposed receiver. Computer simulation results then confirm the correctness of the theoretical analysis and demonstrate that the proposed blind receiver can successfully resist MAI and outperform the conventional receivers.

A Tree-Structured Blind Algorithm for Joint Parametric Channel Estimation and Non-coherent Data Detection in an OFDM-CDMA Wireless System with Antenna Arrays

A blind joint parametric channel estimation and non-coherent data detection algorithm is proposed for the downlink of an orthogonal-frequency-division-multiplexing code-division-multiple-access (OFDM-CDMA) system with multiple-input-multiple-output (MIMO) antenna arrays. To reduce the computational complexity, we first develop a tree-structured algorithm to estimate high dimensional parameters predominantly describing the involved multipath channels by employing several stages of low dimensional parameter estimation algorithms. In the tree structure, to exploit the space-time distribution of the receive multipath signals, spatial beamformers and spectral filters are adopted for clustered-multipath grouping and path isolation. In conjunction with the multiple access interference (MAI) suppression techniques, the proposed tree architecture algorithm jointly estimates the direction of arrivals, propagation delays, carrier frequency offsets and fading amplitudes of the downlink wireless channels in a MIMO OFDM-CDMA system. With the outputs of the tree architecture, the signals of interest can then be naturally detected with a path-wise maximum ratio combining scheme.

Symmetry-Embedded Spreading Sequences for Multicarrier CDMA

It was shown by Slimane that the bit error rate (BER) performance of the downlink of multicarrier code-division multiple-access (MC-CDMA) systems can be improved significantly by employing quadrature spreading sequences instead of binary Walsh-Hadamard sequences. Inspired by Slimane's work, we propose a class of complex-valued spreading sequences, which are mathematically derivable from the Kronecker product of some basic matrices. The good algebraic structure in the proposed spreading sequences, when applied in a BPSK based MC-CDMA system, can be exploited for multiple access interference (MAI) suppression, for order-2 diversity combining, and for the reduction of peak-to-average power ratio. The new spreading sequences outperform the Slimane's quadrature sequences, as evidenced by various numerical results.

Development and Performance Analysis of Non-data Aided MMSE Receiver for DS-CDMA Systems

In this paper, a non-data aided minimum mean square error (MMSE) receiver with enhanced multiple access interference (MAI) suppression is proposed for direct-sequence code-division multiple-access (DS-CDMA) systems over a multipath fading channel. The design of the proposed receiver is via the following procedure: First, an adaptive correlator is constructed based on the linearly constrained minimum variance (LCMV) criterion to collect each multipath signal and suppress MAI blindly. A maximum ratio combiner is then utilized to coherently combine the correlator outputs. With a set of judicious chosen weight vectors, effective diversity combining can successfully suppress MAI and the desired signals can be effectively retained. Finally, further performance improvement against the finite data sample effect is achieved using a decision-aided scheme in which the channel response is obtained by the decision data and incorporated with the MMSE method to compute the refined weight vector. Performance analysis based on the output signal-to-interference-plus-noise ratio (SINR) is done to examine the efficacy of the proposed non-data aided MMSE receiver, which can offer the similar results as those of the MMSE receiver with the channel estimation correctly obtained beforehand. Computer simulation results then confirm correctness of the analysis results and demonstrate that the proposed blind receiver can successfully resist MAI as well as the finite data sample effect, and significantly outperform than the conventional blind receivers.

Constrained Adaptive Constant Modulus RLS Algorithm for Blind DS-CDMA Multiuser Receiver under Time-Varying Channels

The performance of the blind multiuser detector for a DS-CDMA system with linearly constrained constant modulus (LCCM) criterion is known to highly depend on the exact knowledge of the desired user amplitude; it is usually not available at receiver end. In this paper, we propose a novel LC adaptive CM RLS (LC-ACM-RLS) algorithm to adaptively implement the optimal solution of the LCCM receiver, and to track the desired user's amplitude, simultaneously. From computer simulations, we verify the superiority of the new proposed algorithm over the conventional LCCM-RLS algorithm for multiple access interference (MAI) suppression. Also, for time-varying channel during the adaptation processes, if the amplitude of desired user is not available and varies with time, such as hand-off and Rayleigh fading environments, we show that the proposed LC-ACM-RLS algorithm has better tracking capability compared with the conventional approaches.

Supervised Phase Correction of Blind Space–Time DS-CDMA Channel Estimates

Blind channel-estimation algorithms return phase-ambiguous estimates. From a receiver design point of view, the phase-ambiguity problem can be by-passed by differential modulation and detection at the expense of a well-known performance loss, in comparison with direct modulation and coherent detection schemes. An alternative approach is followed in this paper. A theoretical minimum mean-square error phase-estimation criterion leads to a supervised phase-recovery procedure that directly corrects the phase of arbitrary linear filter receivers through a simple closed-form projection operation. Conveniently, any known blind channel-estimation algorithm can be used to provide the initial phase-ambiguous estimate. The presentation is given in the context of adaptive space-time receiver designs for binary phase-shift keying direct-sequence code-division-multiple-access antenna array systems. Numerical and simulation studies support the theoretical developments and show that effective phase correction and multiple-access interference suppression can be achieved with about 2% pilot signaling

Joint Interference Suppression and Frequency Offset Compensation Techniques for a Reverse Link MC-CDMA System

This paper addresses several relevant issues in detection process of a reverse link multicarrier code division multiple access system with frequency offset. Due to the presence of frequency offsets, power of a group of subcarriers is spread to all the available subcarriers. In order to collect all the scattered signal power, we propose a two stages receiver. First stage of the proposed receiver performs maximum ratio combining. Following from the first stage, multiple access interference is suppressed in the second stage. We propose various multiple access interference suppression techniques, having different complexity and performance. Simulation results demonstrate the advantage of the proposed receiver. We also investigate the performance of 2 major subcarrier assignment schemes, namely continuous subcarrier assignment scheme and interleaved subcarrier assignment scheme, in both ideal independent fading channel and practical correlated fading channel.

Multiple-Access Interference Suppression in Multi-User OFDM Systems with Frequency Diversity

This letter deals with our investigations into improving the performance of a wireless uplink system when an orthogonal frequency-division multiple access (OFDMA) is used as an access scheme. To do this, the OFDMA-based uplink system adopts a frequency diversity coupled with a cyclic time shift (CTS) at the transmitter, which is named as the FD-OFDMA system with CTS. It is found that the multi-user FD-OFDMA system equipping with CTS can decrease the probability of destroying the orthogonality among the users and provide the MAI-robust reception without decreasing the bandwidth efficiency of the system.

Improved DSE-CMA error function for blind multiuser interference suppression in DS/CDMA systems

This paper presents an improved error function of dithered signed-error constant modulus algorithm (IDSE-CMA) for blind multiuser interference suppression in DS/CDMA systems. It uses a different error function to replace the former one in sign operation of the DSE-CMA and compares their performance in multiple access interference (MAI) suppression ability. Simulations indicate that the new algorithm has better performance than the similar CMA in terms of convergence speed and steady-state performance.

Efficient Blind MAI Suppression in DS/CDMA Systems by Embedded Constraint Parallel Projection Techniques

This paper presents two novel blind set-theoretic adaptive filtering algorithms for suppressing Multiple Access Interference (MAI), which is one of the central burdens in DS/CDMA systems. We naturally formulate the problem of MAI suppression as an asymptotic minimization of a sequence of cost functions under some linear constraint defined by the desired user's signature. The proposed algorithms embed the constraint into the direction of update, and thus the adaptive filter moves toward the optimal filter without stepping away from the constraint set. In addition, using parallel processors, the proposed algorithms attain excellent performance with linear computational complexity. Geometric interpretation clarifies an advantage of the proposed methods over existing methods. Simulation results demonstrate that the proposed algorithms achieve (i) much higher speed of convergence with rather better bit error rate performance than other blind methods and (ii) much higher speed of convergence than the non-blind NLMS algorithm (indeed, the speed of convergence of the proposed algorithms is comparable to the non-blind RLS algorithm).

Large System Transient Analysis of Adaptive Least Squares Filtering

The performance of adaptive least squares (LS) filtering is analyzed for the suppression of multiple-access interference. Both full-rank LS filters and reduced-rank LS filters, which reside in a lower dimensional Krylov space, are considered with training, and without training but with known signature for the desired user. We compute the large system limit of output signal-to-interference-plus-noise ratio (SINR) as a function of normalized observations, load, and noise level. Specifically, the number of users K, the degrees of freedom N, and the number of training symbols or observations i all tend to infinity with fixed ratios K/N and i/N. Our results account for an arbitrary power distribution over the users, data windowing (e.g., recursive LS (RLS) with exponential windowing), and initial diagonal loading of the covariance matrix to prevent ill-conditioning. Numerical results show that the large system analysis accurately predicts the simulated convergence performance of the algorithms considered with moderate degrees of freedom (typically N=32). Given a fixed, short training length, the relative performance of full- and reduced-rank filters depends on the selected rank and diagonal loading. With an optimized diagonal loading factor, the performance of full- and reduced-rank filters are similar. However, full-rank performance is generally much more sensitive to the choice of diagonal loading factor than reduced-rank performance.

Tradeoff Between Diversity Gain and Interference Suppression in a MIMO MC-CDMA System

In this paper, the uplink of an asynchronous multi-carrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with multiple antennas at both the transmitter and the receiver is considered. We analyze the system performance over a spatially correlated Rayleigh fading channel with multiple-access interference (MAI), and evaluate the antenna array performance with joint fading reduction and MAI suppression. Assuming perfect channel knowledge available at the transmitter, maximal ratio transmission is employed to weight the transmitted signal optimally in terms of combating signal fading. At the receiver, adaptive beamforming reception is adopted to both suppress MAI and combat the fading. Note that while correlations among the fades of the antennas in the receive array reduce the diversity gain against fading, the array still has the capability for interference suppression. We examine the effect of varying the number of transmit and receive antennas on both the diversity gain and the interference suppression.

Subspace-Based Interference Suppression Technique for Long-Code Downlink CDMA Adaptive Receiver

This paper presents a multiple constrained subspace based multiuser detector for synchronous long-code downlink multirate DS-CDMA systems. The novel receiver adapts its fractionally-spaced equaliser tap-weights based upon two modes, namely training and decision-directed modes. Switching between two modes is achieved by changing the code constraint in the associated subspace algorithm. Moreover, detection of the desired user requires the knowledge of the desired user's spreading code only. Simulation results show that the proposed receiver is capable of multiple access interference (MAI) suppression and multipath mitigation. Besides, the results reveal the improvement in terms of convergence speed and mean square error (MSE) of the proposed receiver over the existing receiver in both static and dynamic environments.

Quasi-decorrelating detector: a non-matrix inversion based decorrelating detector with near-far resistance and complexity trade-off

This paper proposes a non-matrix inversion based algorithm to implement decorrelating detection (DD), namely quasi-decorrelating detector (QDD), which uses truncated matrix power expansion instead of inverted correlation matrix to overcome the problems associated with the inversion transformation in DD, such as noise enhancement, computational complexity and matrix singularity etc. Two alternative QDD implementation schemes are presented in this paper; one is to use multi-stage feed-forward filters and the other is to use an nth order single matrix filter (neither involves matrix inversion). In addition to significantly reduced computational complexity, if compared with DD, the QDD algorithm offers a unique flexibility to trade among multiple access interference (MAI) suppression, near-far resistance and noise enhancement according to varying system set-ups. The obtained results show that the QDD outperforms decorrelator in either additive white Gaussian noise (AWGN) or multi-path channel, if the number of feed-forward stages is chosen properly. This paper also studies the impact of correlation statistics of spreading codes on the QDD's performance with the help of a performance-determining factor derived in this paper, which offers a code-selection guideline for the optimal performance of the QDD algorithm. Copyright © 2004 AEI.

Performance Bounds on Chip-Matched-Filter Receivers for Bandlimited DS/SSMA Communications

Performance bounds on chip-matched-filter (CMF) receivers for bandlimited direct-sequence spread-spectrum multiple-access (BL-DS/SSMA) systems with aperiodic random spreading sequences are obtained. First, the optimum transmit-receive chip waveform pairs that maximize the conditional signal-to-interference ratio are derived. This leads to performance bounds on CMF receivers when the conditional Gaussian approximation for cyclostationary multiple-access interference (MAI) is exploited. The bounds are used to examine the dependence of the MAI suppression capability of the CMF receivers on the excess bandwidth of the system and the delay profile of multiple-access users. The system employing the flat-spectrum chip waveform pair is shown to have near-optimum average bit-error rate performance among the fixed CMF (FCMF) receiver systems. Numerical results are provided for an adaptive CMF receiver and for FCMF receivers employing several different fixed chip waveforms.